Passive vehicle occupant restraint systems generally provide a storage capacitor which is connected in parallel with the vehicle battery and provides a secondary source of energy for the firing circuit of the system. Since energy is proportional to the value of the capacitor and the square of the voltage, the size of the capacitors used in prior art systems has been relatively large to ensure a sufficient source of energy in the event of loss of battery during a collision. Such prior art system provide diagnostic circuitry which may include a computer for monitoring the voltage at the firing squib to determine the existence of various potential malfunctions such as open circuit or short to ground or a change in the resistance of the firing circuit.